The present invention is directed generally to a high power semiconductor switch, and particularly to an optically controlled thyristor having optical control.
The need for improved electrical grid power quality and greater utility power control resulted in the implementation of various semiconductor-based electrical control systems such as circuit breakers, volt-ampere reactive (VAR) compensators, and sub-synchronous resonance (SSR) dampers.
The thyristor is a well known power semiconductor switch that permits large currents to be switched at high voltages. The thyristor has four semiconducting layers, Typically, the two outer layers are heavily doped extrinsic layers, while the inner two are lightly doped. Adjacent layers are oppositely doped from their neighbors, forming a number of semiconductor junctions therebetween.
The thyristor is turned on when carriers enter one of the inner layers. Typically, this is performed by injecting a small gate current pulse into one of the inner layers. Where the gate current is injected over only a portion of the inner layer, the current through the thyristor does not reach a maximum value until the entire layer is conducting. The time taken for the current to spread laterally to fill the layer is limited by the lateral carrier diffusion velocity. The device only reaches full current capacity after the carriers have diffused sufficiently to uniformly saturate the device. One approach to reducing the turn on time is to inject the gate current over a large area of the thyristor, which necessitates a high degree of interdigitization between the gate electrode and the cathode electrode. An increased gate electrode results in less active area on the wafer for carrying the high power current.
Another method of turning on a thyristor is to overvolt the device, thus creating carriers by avalanche breakdown in the vicinity of the reverse-biased central junction between the two inner layers. However, this method is less controllable than gate current injection and creates damage regions in the device, thus reducing its useful lifetime.
A third method for turning on a thyristor is to create charge carriers in the center junction through the absorption of light. Past attempts at light activation of a thyristor have included illuminating a portion of the thyristor with light transmitted from the end of a fiber. This resulted in the turn-on time of the thyristor again being limited by lateral diffusion velocity.
The basic thyristor is limited to being actively switched only to the on state, and cannot be actively switched to the off state. Unlike the gate of a transistor, the gate of the thyristor loses active control once the device starts to conduct, and the thyristor switches off only when the current falls below a certain holding current value for a certain minimum time. This inability to be actively turned off limits the use of the thyristor in many applications.
There have been several attempts to make a power semiconductor switch having active on/off switching capabilities. Ideally, semiconductor switches providing active on/off capabilities should have the following characteristics: 1) their power handling capacity should approach that of the thyristor, while maintaining the relatively high speed of metal-oxide semiconductor field effect transistors (MOSFETs); 2) they should have the ability to match the device impedance to the particular application, and thus voltage and current choices should span a broad range without sacrificing other performance parameters; and 3) they should have trigger isolation for series stacking.
There are three existing bipolar semiconductor switches, namely the gate turn-off thyristor (GTO), the insulated gate bipolar transistor (IGBT) and the metal oxide semiconductor (MOS) controlled thyristor (MCT), that are used for on/off switches. All three fall short of the requirements described in the previous paragraph due to their inherent limitations.
The GTO has turn-off capability by externally diverting the emitter current to the gate, thus requiring a large gate area that competes with the emitter area for current-carrying capacity on the wafer. A reduced emitter area results in a reduced current carrying capacity. In addition, there is a large current tail associated with the turn-off. The current pinching substantially reduces the turn-off current relative to the maximum on current. Also, the large gate current of the GTO requires substantial external circuitry.
The IGBT uses a MOSFET to provide a short in the Darlington fashion to supply the base current. The power transmitted by the IGBT is principally limited by the fact that MOSFETs are more limited in their current carrying capability in comparison to bipolar devices. Thus, to accommodate a larger collector current with a small MOSFET current requires a large current gain. To achieve a large gain requires a narrow base region, which increases the emitter injection efficiency. However, the narrow base results in a lower voltage, thereby resulting in a lower power device.
The MCT is a thyristor-based structure that uses MOSFETs to provide electrical shorts for both turn-on and turn-off operations. In turn-off, the current is limited by the MOSFET, which has to fully short one of the injecting diodes of the thyristor structure. Turn-on also is affected by shorting MOSFETs, which necessitates bringing the deep n-drift region of the thyristor structure to the outside surface, thus resulting in a loss of real estate on the wafer which could otherwise be used for actively carrying current. Consequently, the MCT typically falls short of carrying the power levels of the thyristor, the GTO, and the IGBT.
Lastly, the use of MOSFETs to create emitter shorts in the MCT for turn-off requires the MOSFET to compete with the bipolar device in current carrying capability, since the entire bipolar device current is shunted into the MOSFET. If the MOSFET was capable of carrying the full current carried by the bipolar device, there would not be a need for the bipolar device in the first place. The fundamental concept of the MCT is that when the elemental device is shrunk sufficiently, the reduced bipolar current can be carried by the MOSFET during the opening process. Consequently, the current carrying capability is reduced because of the large area allocation for the MOSFET structure. Additionally, there is increased expense resulting from the finer structure design.
To summarize, existing on/off semiconductor power devices fall short in both on-activation and off-activation. Present methods of on-activation demand a compromise between current carrying capacity and device turn on time. Present methods of off-activation are limited due to either the use of MOSFETs to provide shorts (MCT), or due to the limited current and associated speed that can be effectively diverted with external circuits (GTO).
Therefore, there exists a need for an improved semiconductor power switch having on/off control, and whose power handling capabilities in terms of switch response time and total current carrying capacity are increased.